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Full text of "Treatise On Analysis Vol-Ii"

364       XV    NORMED ALGEBRAS AND SPECTRAL THEORY

sequence (xn)n^Q of elements of A, as follows: XQ = x, <p(0) = 0; suppose that
<p(ri) has been defined and that the ek have been defined for 1 g k g (p(ri), and

<p()

let xn = x -  xefc. If jcn = 0, the sequence (ek) is finite and has (p(ri) elements;

fc=i

we take xm = xn = 0 for m ^ n, and <p(w) = <p() for m 2> . If %M ^ 0, take a
finite (possibly empty) sequence ODi^r of irreducible self-adjoint idempo-
tents belonging to I, orthogonal in pairs and orthogonal to each ek for
1 ^ &<;<?(), such that \\xe\\\2  \\x\\2/2n+\ and that the number r is as
large as possible among all finite sequences having the above properties. (We
have seen above that this number is <>2n+*.) Then put <p( + 1) = <p(ri) + r;

r

ek = e\ for fc = cp(ri) + /, 1 < / ^ r; and xn+1 = xw -  *<. If no xn vanishes,

i=l

the sequence ((p(n)) tends to +00; for if it were bounded, we should have
cp(m) = cp(ri) for some n and all m^n, and by definition this would mean that
for each irreducible self-adjoint idempotent e' in the orthogonal supplement

tp(n)

F of  Aek, we should have xe' = xne' = 0; but xn E F, xn = 0, and F is a

fc=i
closed left ideal (15.8.2), so that this would contradict what was established at

the beginning of the proof.

This being so, it is clear from the construction that  \\xen\\2 ^ \\x\\2 by

n

virtue of (15.8.9); hence if the series  xen is not a finite sum, it converges in

n

any case to an element y e I which is the orthogonal projection of x on the
closure of the left ideal a which is the sum of the Aen (for x  y, being

m

the limit of x  xek, is orthogonal to all the en) (6.5.2). If x  y ^0,

*=i
there would exist in the left ideal a1 an irreducible self-adjoint idempotent

e" e I such that xe" -(x- y)e" ^ 0. If n is the smallest integer such that
\\xe"\\2 ^ ||jc||2/2"+1, the existence of e" would contradict the maximality of
the family of et such that <p(n) < i ^ cp(n + 1).                                   Q.E.D.

(15.8.11) Suppose that the algebra A is separable. Then every closed left ideal
b is the Hilbert sum of a (finite or infinite) sequence of minimal left ideals
ln = Aen, where en is an irreducible self-adjoint idempotent. For each x e b we
have x = ^ xen, and for all x, y in b we have (x\y) =  (xen \ yen).

The second and third assertions are consequences of the first and of the
definition of a Hilbert sum (6.4), since xen is the orthogonal projection of
x on Aen (15.8.9). To prove the first assertion, let (xn)n^i be a dense sequence
in b (3.10.9). We define inductively, for each , a (finite or infinite) sequence
(en, i)iein of irreducible self-adjoint idempotents, as follows. For (elti)ielnn A2||x||- \\(x*?\\ <p(x*x)n for all